One of the reasons I chose the 6.4L over the 6.7L was the significant power increase with just a few simple mods with the 6.4L. Since we are talking diesel trucks I will say that having 550rwhp and 1150ft-lbs (not crank numbers from a marketers pamphlet btw) with power levels controlled from inside the cabin is nice. It takes a significant amount of work to get the others there. FWIW I'm not against any manufacturer or engine... I just spent my money on what I like and what I wanted and I'm sure you all do the same.
The other reason was an additional $8000 off in rebates not offered on the newer model at the time. Money talks.